ALKALI CONDITIONS AT FOUGALA 



81 



It will be noted that the most readily soluble salts, sodium sulphate and the 

 chlorides, are largely concentrated in the surface soil. This is shown graphically in 

 the accompanying diagram (fig. 9), in which the curves are smoothed so as to show 

 approximately the distribution at various depths of the more important salts com- 

 posing the alkali at this station. 



This distribution of alkali is the common one when there is an appreciable rain- 

 fall, as in the northern Sahara (about 9 inches at Biskra), but is very unlike that of 

 the nearly rainless Salton Basin, where the subsoil often contains more alkali than 

 the surface layers. 



This soil was excessively alkaline, the surface foot containing nearly one-twentieth 

 of its weight of alkali, and the whole surface soil to a depth of 4 feet containing nearly 

 2.5 per cent of alkali. Thealkali is characterized by the large proportion of chlorides 

 (amounting to 81 per cent of the total salts), of which almost all is common salt, 

 which alone makes up nearly 4 per cent of the weight of the surface foot, or some 

 160,000 pounds per acre in the surface foot! 



Common, S 'alt: fSodium, Chlorid ) 



Magnesium, Svdphate 



Glcuuber* SaJbL-(So(luum, Svuiphate) 



FIG. 9. Curves showing distribution of alkali to a depth of 4 feet in uncultivated Saharan soil at 



Station No. 1, Fougala, Algeria. 



This soil is very interesting as representing practically the extreme limit of endur- 

 ance of the date palm for this type of alkali. Unfortunately samples were not 

 obtained down to the hardpan, but if the decrease followed the same ratio as in the 

 Station No. 2, the amount of alkali in the subsoil at 30 to 48 inches would be about 

 1.42 per cent, and the average for the soil to a depth of 4 feet, 2.55 percent. 



Station No. 2, where the soil was sampled at Fougala, was only a few hundred 

 feet from Station No. 1, in a young date plantation, where irrigation had been prac- 

 ticed for three years. The samples were taken by cutting away a foot or so of the 

 side of the pit, or " bir," in which a date palm had been planted three years before. 

 Fresh earth was reached before the sample was taken. Hardpan was encountered 

 at a depth of 4 feet. The appearance of the locality is shown in the background of 

 Plate XV, figure 2. 



13529 No. 5304 6 



